T. F. Thingstad

Counterintuitive carbon-to-nutrient coupling in an Arctic pelagic ecosystem

Predicting the ocean’s role in the global carbon cycle requires an understanding of the stoichiometric coupling between carbon and growth-limiting elements in biogeochemical processes. A recent addition to such knowledge is that the carbon/nitrogen ratio of inorganic consumption and release of dissolved organic matter may increase in a high-CO2 world. This will, however, yield a negative feedback on atmospheric CO2 only if the extra organic material escapes mineralization within the photic zone. Here we show, in the context of an Arctic pelagic ecosystem, how the fate and effects of added degradable organic carbon depend critically on the state of the microbial food web. When bacterial growth rate was limited by mineral nutrients, extra organic carbon accumulated in the system. When bacteria were limited by organic carbon, however, addition of labile dissolved organic carbon reduced phytoplankton biomass and activity and also the rate at which total organic carbon accumulated, explained as the result of stimulated bacterial competition for mineral nutrients. This counterintuitive ‘more organic carbon gives less organic carbon’ effect was particularly pronounced in diatom-dominated systems where the carbon/mineral nutrient ratio in phytoplankton production was high. Our results highlight how descriptions of present and future states of the oceanic carbon cycle require detailed understanding of the stoichiometric coupling between carbon and growth-limiting mineral nutrients in both autotrophic and heterotrophic processes.

Are viruses important partners in pelagic fend webs

Viruses have been assumed to play a rather negligible role as partners in microbial food web dynamics. However, recent discoveries suggest that the rate of virally induced lysis of marine microbial populations may be significant. This, in turn, may have important consequences for the developing conceptual framework of the microbial food web.

A theoretical analysis of how strain-specific viruses can control microbial species diversity.

Significance This work presents the first detailed analysis to the authors’ knowledge of how species-level diversity is a property emerging from competitive and defensive abilities at the organism level in a microbial system where the diversity-generating mechanism is strain-specific viral lysis. The theoretical analysis constitutes a general case treatment of the important special case question of what properties may make SAR11, a subphylum within the Alphaproteobacteria, so dominant in the pelagic environment. The resulting conceptual framework connects differences in the molecular defense mechanism to ecosystem-level properties such as diversity and activity. It also suggests a reinterpretation of the concept of dormancy in aquatic microbial communities. Pelagic prokaryote communities are often dominated by the SAR11 clade. The recent discovery of viruses infecting this clade led to the suggestion that such dominance could not be explained by assuming SAR11 to be a defense specialist and that the explanation therefore should be sought in its competitive abilities. The issue is complicated by the fact that prokaryotes may develop strains differing in their balance between competition and viral defense, a situation not really captured by present idealized models that operate only with virus-controlled “host groups.” We here develop a theoretical framework where abundance within species emerges as the sum over virus-controlled strains and show that high abundance then is likely to occur for species able to use defense mechanisms with a low trade-off between competition and defense, rather than by extreme investment in one strategy or the other. The J-shaped activity–abundance community distribution derived from this analysis explains the high proportion low-active prokaryotes as a consequence of extreme defense as an alternative to explanations based on dormancy or death due to nutrient starvation.

Strong Seasonality and Interannual Recurrence in Marine Myovirus Communities

ABSTRACT The temporal community dynamics and persistence of different viral types in the marine environment are still mostly obscure. Polymorphism of the major capsid protein gene, g23, was used to investigate the community composition dynamics of T4-like myoviruses in a North Atlantic fjord for a period of 2 years. A total of 160 unique operational taxonomic units (OTUs) were identified by terminal restriction fragment length polymorphism (TRFLP) of the gene g23. Three major community profiles were identified (winter-spring, summer, and autumn), which resulted in a clear seasonal succession pattern. These seasonal transitions were recurrent over the 2 years and significantly correlated with progression of seawater temperature, Synechococcus abundance, and turbidity. The appearance of the autumn viral communities was concomitant with the occurrence of prominent Synechococcus blooms. As a whole, we found a highly dynamic T4-like viral community with strong seasonality and recurrence patterns. These communities were unexpectedly dominated by a group of persistently abundant viruses.

Elemental composition of individual pico- and nano-sized marine detrital particles in the northwestern Mediterranean Sea

The elemental composition of individual < 10 μm detrital particles from Mediterranean surface waters was analysed using a Transmission Electron Microscope (TEM) equipped with an energy dispersive X-Ray microanalyser. Results show that carbon and phosphorus content per detritus volume are much higher in pico-detrital particles < 2 μm (42 kg C m−3 and 1 kg P m−3) than in 5–10 μm detrital particles (20 kg C m−3 and 0.1 kg P m−3). The C:N:P atomic ratios for different sized fractions of the detrital particles were found to be 82:10:1 for < 2 μm particles, 120:29:1 for 2–5 μm particles and 308:37:1 for 5–10 μm particles. The average ratio for all size classes of detrital particles (< 10 μm) was 132:23:1. The differences in elementary compositions of the detrital particles studied here suggest that the different size fractions probably have different origins. The role and origins of < 10 μm detrital particles within the biogeochemical cycles are discussed.